Controlling the characteristics of implanter ion-beams

ABSTRACT

A method and apparatus satisfying growing demands for improving the precision of angle of incidence of implanting ions that impact a semiconductor wafer and the precision of ribbon ion beams for uniform doping of wafers as they pass under an ion beam. The method and apparatus are directed to the design and combination together of novel magnetic ion-optical transport elements for implantation purposes. The design of the optical elements makes possible: (1) Broad-range adjustment of the width of a ribbon beam at the work piece; (2) Correction of inaccuracies in the intensity distribution across the width of a ribbon beam; (3) Independent steering about both X and Y axes; (4) Angle of incidence correction at the work piece; and (5) Approximate compensation for the beam expansion effects arising from space charge. In a practical situation, combinations of the elements allow ribbon beam expansion between source and work piece to 350 millimeter, with good uniformity and angular accuracy. Also, the method and apparatus may be used for introducing quadrupole fields along a beam line.

CROSS REFERENCE AND RELATIONSHIP

This application is a Continuation of U.S. Ser. No. 10/619,702 filedJul. 15, 2003, now U.S. Pat. No. 6,933,507 which claims priority to U.S.provisional patent application Ser. No. 60/396,322 filed Jul. 17, 2002,the disclosures of which are incorporated herein by reference.

FIELD OF INVENTION

The disclosed methods and apparatus relate generally to the constructionand use of magnetic focusing and correction elements for modifying theintensity distribution of ions within ribbon beams and more particularlyto precision correction of the angle of incidence of ions used forimplanting and doping in semiconductor devices.

BACKGROUND TO THE INVENTION

The process of ion implantation is useful in semiconductor manufacturingas it makes possible the modification of the electrical properties ofwell-defined regions of a silicon wafer by introducing selected impurityatoms, one by one, at a velocity such that they penetrate the surfacelayers and come to rest at a specified depth below the surface. It makespossible the creation of three-dimensional electrical circuits andswitches with great precision and reproducibility.

The characteristics that make implantation such a useful processingprocedure are threefold: First, the concentration of introduced dopantatoms can be accurately measured by straight-forward determination ofthe incoming electrical charge that has been delivered by charged ionsstriking the wafer. Secondly, the regions where the above dopant atomsare inserted can be precisely defined by photo resist masks that makepossible precision dopant patterning at ambient temperatures. Finally,the depth at which the dopant atoms come to rest can be adjusted byvarying the ion energy, making possible the fabrication of layeredstructures. Systems and methods are desired for enhancing the ionimplantation process.

The ion species presently used for silicon implantation include arsenic,phosphorus, germanium, boron and hydrogen having energies that rangefrom below 1 keV to above 80 keV. Ion currents ranging from microamperesto multi-milliamperes are needed. Tools providing implant currentsgreater than ˜5 mA are commonly referred to as ‘high-current’implanters. Trends within the semiconductor industry are moving towardsimplantation energies below 1 keV and control of angle of incidencebelow 1°.

Typically, an ion implanter for introducing such dopant materials intosilicon wafers and other work pieces may be modeled into four majorsystems: First, an ion source where the charged ions to be implanted areproduced. Secondly, an acceleration region where the energy of the ionsis increased to that needed for a specified implant procedure. Thirdly,an optical ion transport system where the ion ensemble leaving thesource is shaped to produce the desired implant density pattern andwhere unwanted particles are eliminated. Finally, an implant stationwhere individual wafers are mounted on the surface of an electrostaticchuck or a rotating disc that is scanned through the incoming ion beamand where a robot loads and unloads wafers. One aspect of the presentinvention aims towards enhancing or improving ion beam transportsystems.

A recent improvement for ion implanter design has been the introductionof ribbon beam technology. Here, ions arriving at a work piece areorganized into a stripe that coats the work piece uniformly as it ispassed under the ion beam. The cost advantages of using such ribbon beamtechnology are significant: For disc-type implanters, ribbon-beamtechnology eliminates the necessity for scanning motion of the discacross the ion beam. For single-wafer implanters the wafer need only bemoved under the incoming ribbon beam along a single dimension, greatlysimplifying the mechanical design of end-stations and eliminating theneed for transverse electromagnetic scanning. Using a correctly shapedribbon beam, uniform dosing density is possible across a work piece witha single one-dimensional pass.

The technical challenges of generating and handling ribbon beams are nontrivial because the ribbon beam/end station arrangement must producedose uniformities better than 1%, angular accuracies better than 1degree and operate with ion energies below 1 keV. U.S. Pat. No.5,350,926 entitled “High current ribbon beam ion implanter” and U.S.Pat. No. 5,834,786, entitled “Compact high current broad beam ionimplanter”, both issued to White et al., present some features of ribbonbeam technology.

White et al. have also reviewed some of the problems of generatingribbon beams in an article entitled “The Control of Uniformity inParallel Ribbon Ion Beams up to 24 Inches in Size” presented on page 830of the 1999 Conference Proceedings of Applications of Accelerators inResearch and Industry”, edited by J. L. Dugan and L Morgan and publishedby the American Institute of Physics (1-56396-825-8/99).

By its very nature, a ribbon beam has a large width/height aspect ratio.Thus, to efficiently encompass such a beam traveling along the Z-axis, afocusing lens for such a beam must have a slot-like characteristic withits slot extending along the X-axis and its short dimension across theheight of the ribbon (the Y-direction). The importance of this is that,while the focal lengths of a magnetic quadrupole lens in each dimensionare equal but of opposite sign, the angular deflections of the ribbon'sboundary rays in the width and height dimensions can be very different.In addition, the magnetic field boundaries of the lens can be close tothe ion beam permitting local perturbations introduced along theseboundaries to have deflection consequences that are effectively limitedto a small region of the ribbon beam.

SUMMARY

While in principle it is feasible to generate a wanted shape of ribbonbeam directly from an ion source, in a practical situation full-lengthribbon extraction may not be feasible. Often it is desirable to generatea modest-length ribbon at the source and expand it to the width requiredfor implantation, using ion-optical expansion. Another aspect of thepresent invention is directed towards extracting ions from an ion sourcein the form of a multiplicity of individual beamlets whose centraltrajectories are parallel and arranged in a linear manner. Such geometryprovides a precise definition of the origin and angular properties foreach beamlet. Those skilled in the art will recognize that theseprinciples remain valid even if multiple parallel rows of beamlets areused or if the central trajectories of the beamlets are not parallelwhen they leave the source region or if a slit-geometry is chosen forion extraction.

Furthermore, those skilled in the art will recognize that focusing anddeflection elements will be needed to transport the ions between an ionsource and a work piece where the particles are to be implanted. Forfocusing lenses to operate as ideal focusing elements it is desirablethat, to first order, the angular deflection introduced to thetrajectory of individual beamlets be proportional to the beamletsdistance from the lens symmetry axis; namely, the magnitude of thedeflecting fields should increase linearly with distance from thecentral trajectory of the ion beam.

Quadrupole lenses satisfying the linearity requirement described aboveand having high length to height aspect ratio have been described by W.K. Panofsky et al. in the journal Review of Scientific Instrumentsvolume 30, 927, (1959), for instance. Basically, their design consistsof a rectangular high permeability steel frame with each of the longsides of the frame supporting a single uniformly wound coil. To generatea quadrupole field the top and bottom coils are wound equally spacedalong each of the long sides of the steel frame members with thecurrents through the coils being excited in opposite directions whenviewed from one end of the rectangular array. A north pole at the end ofone bar sees a south pole facing it. On the short sides of therectangular frame, additional coils are used to buck the magnetostaticpotential at both ends of each long side preventing magnetic shortcircuits through the end-bars. For quadrupole field generation theopposing ampere-turns along each vertical bar are equal to theampere-turns along each of the long bars. The currents passing throughthese two bucking coils will be equal but generate fields in opposingdirections.

For many focusing applications the correction of aberrations and thecompensation of non-linear spreading of a low energy beam is critical sothat the possibility for producing deviations from a linear growth ofmagnetic field away from the center is desirable. A method forintroducing the necessary multipole components to the field has beendescribed by Enge '328 in U.S. Pat. No. 3,541,328, particularly, themethod described in this document for producing multipole focusingfields in the space between two iron cores between which ions arepassed. A series of independently excitable windings, each having a coildistribution appropriate for generating a specific multipole, are woundalong each of the iron cores. In the journal Nuclear Instruments andMethods, volume 136, 1976, p213-224 H. J. Scheerer describes thefocusing characteristics of such a dual rod design in accordance withthe description in U.S. Pat. No. 3,541,328. Specifically, in FIG. 6 ofthis patent it can be seen the coils for each multipole are connected inseries and powered as a single unit.

The Panofsky quadrupoles and Enge multipole generators were bothconceived for transmitting ions through a beam transport system wherethe parameters of the ion transport elements are fixed for a singleexperiment or measurement. They suffer disadvantages when active controlof the deflecting fields is needed to correct beam parameters. First,neither design generates a dipole field contribution where the B-fieldis along the long axis of the rectangle. Secondly, the symmetry point(x=0) is usually established from the geometry of the coils and of thesteel yokes so there is no easy way to introduce steering about they-axis by moving the center of the lens-field distribution.

In an embodiment of the present invention, a rectangular steel windowframe construction provides the magnetic supporting structure needed forproducing the wanted deflection fields. A feature of the presentembodiment is that the windings along the long-axis bars consist of alarge number of independently excited short sections. This conceptallows high-order multipoles to be generated without dedicated windingsand the central point of any multipole contribution can be translatedalong the transverse x-axis. Additional coils around the end bars areessential for eliminating magnetic short circuits when multipolecomponents are being generated. However, these end-bar coils can also beexcited independently in a manner that allows the production of a puredipole field between the long-axis bars at right angles to the longdimension of the rectangle. Finally, when the coils on the end bars areswitched off, dipole fields can be generated along the long axis of thewindow frame.

In another embodiment of the present invention, local variations in iondensity or the shape of the ribbon beam at the exit from the source arecorrected by locally modifying the deflecting fields. These correctionscan be made under computer control and on a time scale that is onlylimited by the decay rate of eddy currents in the steel. The input beamparameters needed for control involves position-sensitive faraday cupsfor measuring the intensity and angle distribution of ions within saidribbon beam allowing discrepancies from the wanted distribution to becorrected by modifying the deflection fields.

While each of the applications of such lens variations will be discussedfurther, it should be appreciated that, because of linear superpositionof fields in free space, the currents necessary to produce a particulartype of correction can be calculated individually. This process can berepeated for each type of correction needed with the complete solutionbeing produced by superposition. Such concurrent introduction of aselected group of multipole fields into a single beam transport elementhas been described by White et al. in the journal Nuclear Instrumentsand Methods volume A 258, (1987) pp. 437-442 entitled “The design ofmagnets with non-dipole field components”.

The fundamental concept underlying the present invention is the creationof a region filled with magnetic fields that encompasses alltrajectories comprising a ribbon beam. The d.c. magnetic fields having amagnitude and direction throughout the region that is appropriate tointroduce the wanted deflections of all beamlets constituting the ribbonbeam. Within the constraints implied by Maxwell's equations, magneticfield configurations can be chosen that provide controlled changes inthe angular coordinates of beamlets and produce superposed correctionsfor: (1) angular errors, (2) differential intensity errors, (3) uniformsteering about axes normal to both (y₀, z₀) and (x₀, z₀) planes, (4) theintroduction of linear positive and negative focusing, (5) specializeddeflection fields for aberration correction.

Other objects and advantages will become apparent herinafter in view ofthe specifications and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

For better understanding of the present invention, reference is made tothe accompanying drawings, which are incorporated herein by reference

FIG. 1 illustrates a beam coordinate system used in connection with anembodiment of the present invention;

FIG. 2 illustrates expansion optics for an optical expander used inconnection with an embodiment of the present invention;

FIG. 3 illustrates the geometry of a lens corrector according to anembodiment of the present invention;

FIG. 4 illustrates a cross sectional view of FIG. 3 in the x-directionshowing how magnetostatic potential transfer plates transfer potentialto the region of the ribbon beam according to an embodiment of thepresent invention;

FIG. 5 Illustrates an enclosure for a lens corrector according to anembodiment of the present invention;

FIG. 6 Illustrates a focusing lens and corrector assembly according toan embodiment of the present invention;

FIG. 7 illustrates quadrupole operation according to an embodiment ofthe present invention;

FIG. 8 illustrates the magnetostatic potentials that are needed togenerate a quadrupole magnetic field in an embodiment of the presentinvention;

FIG. 9 a illustrates changes in magnetostatic potential with resultantchanges of beam width of a ribbon beam for an embodiment of the presentinvention;

FIG. 9 b illustrates ribbon beam expansion/contraction in connectionwith the magnetostatic potential distribution shown in FIG. 9 a inconnection with an embodiment of the present invention;

FIG. 10 a illustrates magnetostatic potential correction associated witha change in the local ribbon density in connection with an embodiment ofthe present invention;

FIG. 10 b illustrates a local ribbon beam intensity correction inconnection with steering about the y-axis in connection with anembodiment of the present invention;

FIG. 11 a illustrates magnetostatic potential for introducing y-steeringof a ribbon beam in connection with an embodiment of the presentinvention;

FIG. 11 b illustrates a beam motion in the x direction at the wafersteering or steering about the y-axis of a ribbon beam in connectionwith an embodiment of the present invention;

FIG. 12 illustrates the mode of steering of a ribbon beam about they-axis in connection with an embodiment of the present invention;

FIG. 13 a illustrates magnetostatic distribution along both bars inconnection with a mode that allows deflection about the x-axis inconnection with an embodiment of the present invention;

FIG. 13 b illustrates steering about the x-axis of a ribbon beam inconnection with an embodiment of the present invention;

FIG. 14 illustrates a topologically equivalent geometry for alens/steerer corrector element in connection with an embodiment of thepresent invention;

FIG. 15 illustrates principles of a hydrogen ion implanter designed inconnection with an embodiment of the present invention; and

FIG. 16 illustrates a cross section of a composite bar in connectionwith an embodiment of the present invention.

DETAILED DESCRIPTION

The unique properties of the system according to the present inventionwill be better elucidated by reference to a practical example. In thisexample, a pair of quadrupole lenses are used to expand an initiallyparallel set of beamlets to a broader set of parallel beamlettrajectories.

FIG. 1 illustrates the beam coordinate system used in the followingdiscussions. Three representative sections, 120, across a ribbon beamare shown. The X-axis is always aligned with the surfaces, 120, at rightangles to the beamlets, 130, comprising the ribbon beam and along thesurface's long axis. The Z-axis, 110, is tangential to the centraltrajectory, of the ribbon beam and remains coincident with the centraltrajectory throughout the length of the ion optical transport system,causing it to change direction as the central trajectory, 110, changesdirection. At each point along the beam path the Cartesian Y-axis liesalso in the surface, 120, and along the ribbon beam's cross-sectionalnarrow dimension.

FIG. 2 shows the essential structure of an ion beam expander, 200, thatoptically couples an ion source, 201, having narrow width, to produce aribbon height at a work piece or wafer, 220, that allows simultaneousribbon beam implantation across the whole wafer width in a singletraverse of the wafer 220, using linear reciprocating motion, 221. Ashort ribbon beam generated by the ion source 201, in the form a groupof beamlets arranged in a linear array, 210, is expanded so that itswidth at a converging lens, 250, matches that needed at a work piece,220, being implanted. The beam expander, 200, further comprises adiverging lens, 230, followed by a free-space drift region, 240, wherethe individual ion beamlets drift apart before they are collimated backto parallelism by the larger width converging lens, 250.

In the preferred embodiment the work piece, 220, passes under anexpanded ribbon beam pattern, 260, at constant velocity with the angleof incidence being adjustable by rotating the wafer about an axis, 270,to modify the ion impact angle, θ. When the wafer is rotated about theaxis, 270, to large angles, the beam width can be adjusted by modifyingthe expansion ratio to minimize beam wastage. For the geometry of FIG. 2the ion density should be constant across the width of the ribbon beam.However, for geometries such as those of a rotating disc type implanter,the ion density within the ribbon beam must vary with implant radius. Inthis case, it will be clear that to produce doping uniformity at thework piece the ribbon beam ion density will generally require activecorrection across the ribbon beam.

FIG. 3 shows the basic features of lens correctors according to theembodiment of the present invention. A high-permeability rectangularsteel structure, 310, aligned with its long axis parallel to the widthof a ribbon beam, 320, (X-coordinate) and with its geometric centercoincident with the geometric center of the ribbon beam, supports coils,330, 340, that are used to generate the wanted magnetic fields within agap, 312, through which the ions forming the ribbon beam, 320, aredirected. Individual coils, 330, 340, shown schematically, aredistributed along both long-axis bars, 314, 316, of the rectangularsteel structure, 310, with individual controllable power suppliesestablishing the current through each of the coils via the circuits, 350and 351. While, for clarity, the individual coils, listed as 330 and340, are shown with considerable separation, in practice the coilsshould be as close together as is practical to allow the magnetic fieldon the axis of beam region, 322, to vary smoothly. For some applicationswhere the coils, 330, and 340, must have large cross section to minimizepower dissipation, thin ferromagnetic plates (not shown) can be used toseparate individual coils and relay the scalar potentials nearer to theion beam boundaries. Alternatively, the coils 330 and 340 may beconnected together as a continuous coil.

End coils, 332 and 342, shown in FIG. 3, are not necessarily dividedinto multiple elements. Their primary function is to establishappropriate magnetostatic potentials that prevent magnetic shortcircuits between the upper and lower steel bars, 314, and 316. Duringquadrupole operation equal and opposite ampere-turns must be generatedby coils, 332 and 342, to the ampere turns applied along the long axesof the rectangular structure. To make possible the production of severaldeflection modes the current directed through the end coils, 332 and342, should be reversible and adjustable with precision. During thegeneration of dipole magnetic fields along the X-axis, coils 332 and342, may be turned off.

FIG. 4, illustrates a cross-section as viewed along the line A-A′, inthe x-direction, shown in FIG. 3 with the addition of a surroundingvacuum enclosure. It can be seen that small high permeability steeltabs, 420 and 422, mentioned earlier, transfer the magnetostaticpotential generated along each bar, 314 and 316, to the boundaries ofthe ion beam region, 322. The straight section of the steel tabs, 420and 422, should be located as close as possible to the ion beam tolocalize the position resolution of correcting field components.

Without reservations, the projections shown in FIG. 5 show the preferredembodiment of a lens-corrector enclosure. The design goal for theenclosure is to avoid exposure of the vacuum environment to the coilsand their insulation. Also, to avoid vacuum to air feed-throughs forpower feed and water-cooling channels. Basically, a magneticlens/corrector can operate at ambient atmospheric pressure inside suchan enclosure, 510. It has vacuum on the outside, 500, and ambientatmospheric pressure or liquid cooling on the inside, 510. The enclosuremust have a depth along the Z-axis adequate to contain a coil structureas described in FIGS. 3 and 4 and sufficient magnetic path length alongthe ion beam that the ions can be deflected through the wantedcorrection angle. While those skilled in the art will recognize thatthere are many methods of fabricating the enclosure, 510, in the presentembodiment the enclosure is machined from a suitable block of aluminumjig-plate. During operation the enclosure, 510, is bolted to a housingthat is part of an implantation system's vacuum envelope, 530. Such aconstruction serves to define the position of the corrector element withrespect to other optical elements that are part of the beam transportcomponents used in an implanter. The corrector lens shown in FIG. 3 or 4may be connected to the ambient atmosphere via connecting holes, 540 and542. Through these holes, 540 and 542, pass electric power leads foreach of the coils plus air or liquid cooling for the coils. Theenclosure, 510, is made vacuum tight by attaching a simple plate, 550,to the flat surface, 560, sealed with O-rings, 552.

The cross section view of FIG. 6 illustrates an assembled structure of atypical lens-corrector, 600, where like elements are described inprevious embodiments. The rectangular high permeability bar structure,314 and 316, is the basis of the rectangular window frame. It will beseen that for ease of wiring and cooling the steel bars may befabricated from appropriate steel tubing that will allow easy access forthe wiring and cooling lines. The Z-axis of the ribbon beam plane passesthrough the open center, 322, of the corrector. Power and cooling areintroduced through the penetrations, 542. The electrical connections arearranged using the distribution panel, 610.

FIG. 7 illustrates the background to the generation of a quadrupolefield in the region between rectangular bars, 314 and 316, and how sucha distribution can be modified to correct for aberrations. Assuming thata uniform current sheet, j_(z)(x), 701, 702, is produced as illustratedby the modules around the surface of each bar, these current sheets willgenerates a magnetic field, B_(x)(x), in the immediate surface of thewinding given byB _(x)(x)=μ₀ ·j _(z)(x)  (1)

To generate a pure quadrupole field, j_(z)(x) is constant for all valuesof x. Applying Ampere's theoremB _(y)(x)=(μ_(0/d))·j(x)·x  (2)Where d is the distance from each bar to the center line, 710.

Thus, for uniform currents flowing in the manner shown by the arrows inFIG. 7 a north pole generated at the end of one bar sees a south poleimmediately opposite on the adjacent steel bar with the magnetic fieldB_(y)(x) being zero at the center of the x-dimension, measured betweenthe vertical steel connecting bars, 721, 722, and increasing linearlyfrom the center to each end changing sign at the center.

Those skilled in the art will recognize, because of superposition, thatwithin the resolution limit of the geometry and assuming no saturationof the steel, whatever multipole is required can be excited by choosingthe appropriate distribution of the current density, j(x). Clearly,individual windings having constant current and variable pitch canprovide the needed variations in j(x) as has been disclosed in U.S. Pat.No. 3,541,328. However, it is realized that whatever multipole is neededcan be excited by using a single group of windings provided the singlewinding layer is divided into a large number of short individuallyexcited coils, 330 and 340, as illustrated in FIG. 3.

Some Specific Geometries

FIG. 8 is a graphical representation for understanding the generation ofmultipole fields that can be introduced by a lens corrector according tothe embodiment of the present invention. Because excitation currents ared.c., or do not change rapidly with time, it is unnecessary to includevector potentials in the field description. Such a simplification allowsthe use of magnetostatic potentials, alone, for calculating the magneticB-fields (the magnetic induction). The usefulness of this approach isthat under these conditions the same equations are satisfied formagnetostatic fields as are satisfied for electrostatic fields with thedriving potential for magnetostatic fields being ampere-turns ratherthan volts. However, it should be emphasized that such an analysis mustnot include the regions of current excitation which surrounds individualsteel bars. Referring to equation (2) it can be seen that for quadrupolegeneration the difference between the magnetic potentials generatedalong each bar increases linearly from one end of the lens to thedistant end. Thus, assuming uniformly spaced windings and equal currentsthrough each winding, the loci of the associated magnetostaticequipotentials along each bar are straight lines that pass through zeroat the center of each bar, because of symmetry. The B_(y)(x) fields,which are produced between the bars, 314 and 316, described in FIG. 3,are excited by the negative gradient of the magnetostatic potentialdifference. As the distance between the high permeability steel tabs,420 and 422, described in FIG. 4, is constant along the width of thelens/corrector, the difference between the magnetostatic potentials ofeach bar allows B_(y)(x) to be calculated directly.

Using this same presentation, FIGS. 9 a and 9 b show schematically themanner in which expansion (or contraction) of a ribbon beam ensemble canbe accomplished. In FIG. 9 a the magnetostatic equipotentials, 910 and912, associated with a diverging lens, 930, in FIG. 9 b produce areduced-size ribbon beam, 950, starting from a fully expanded beam, 960,produced by equipotentials, 920 and 922. A simple linear change of allof the currents through all of the elementary coils, 330 and 340, allowsexpansion of the width of the ribbon beam to appropriate size before theribbon beam impacts the wafer, 970.

In FIGS. 10 a and 10 b, an individual beamlet, 980, is assumed to leavean ion source, 901, with intensity lower than anticipated for theremainder of the beamlets. To compensate for the reduced local iondensity in the ribbon beam the fan-out pattern produced by the diverginglens, 930, is locally compressed around the attenuated beamlet, 980, byreducing the angular spacing between trajectories, 982, and 984. Whensatisfactory uniformity has been achieved at the entrance to lens 940,the overall spread of the fan is modified, as shown in FIGS. 9 a and 9b, to allow uniform implantation of the whole work piece. It can be seenfrom the magnetostatic potential plot that for both bars forming thediverging lens, 930, the magnetostatic potentials, 924 and 926, nolonger increase linearly from the center of each bar but rather has beenreduced locally, at 925 and 927, to introduce a non-linearity indeflection angles for trajectories 984 and beyond that restoresuniformity of implant intensity along the width of the ribbon beam. Ifnecessary, angle corrections to compensate for this non-lineardeflection can be introduced in lens, 940.

There is a one-to-one correspondence between position along the finalribbon beam and the coil location along the first quadrupole barallowing the computer correction algorithm to be simple and straightforward.

FIGS. 11 a and 11 b show a method for introducing ribbon beam shiftsalong the x-direction or a rotation around the y-axis normal to the X-Zplane of FIG. 11 b. Basically, to introduce a parallel shift all of theindividual coils along both bars of the lens/corrector, 930, areelectrically energized to produce a zero, 990, that is offset from thenominal center of the lens, 930. A compensating correction needed forthe lens 990. To produce rotation about the y-axis the collimatingcurrents through the lens 940, are adjusted appropriately to not returnthe output trajectories to being parallel to the ions leaving thesource, 901.

The principles used for producing the above offset in an alternateembodiment of the present invention are illustrated in FIG. 12. Thecoils, 330 and 340, illustrated in FIG. 3 and distributed along thebars, 314, and 316, are not energized and are left from the drawing tominimize confusion. The bucking coils, 332, 342, produce a uniform stripof magnetic B_(y)-field, 328, that in the median plane is whollyparallel to the direction of the y-axis. Thus, there is no B_(x)-fieldcomponent along the x-direction so that it is not possible to inducemotion out of the X-Z plane. Steering about the Y direction is fullydecoupled from lens action and steering about the X-direction.

FIGS. 13 a and 13 b, show a method for generating uniform B-fields alongthe x-direction. In FIG. 13 a a pair of magnetostatic potentials, 1310and 1316 are generated each having equal magnitude and direction alongthe individual bars with respect to one end. This can be achieved byenergizing the coil collection, 330 and 340, shown in FIG. 3, uniformlyand with the same hand. While the contribution to the magnetostaticpotential from both bars would ideally be equal, it is possible for themto be unequal, as is shown in FIG. 13 a.

In practice, without exceptions, superposition allows all of thesepreviously described field arrangements to be added together to producea combination deflection structure that produces focusing, correctionsof aberrations, corrections for differential variations in sourceoutput, and local steering across the ribbon ion beam around both X andY axes. The constraint is that saturation should be minimal in theferromagnetic members.

A Useful Lens/Corrector Geometry

FIG. 14 illustrates the design of a lens/corrector assembly consistingof two independent elements, 1430 and 1431, between which a ribbon beamcan be directed through the slot, 322. Such a lens/corrector assembly,which is topologically identical to the rectangular steel bar structureillustrated in FIG. 3, has useful characteristics for insertion into thevacuum region of a beam-transport pipe and into the fringe field regionsof a magnetic deflector where the vertical steel parts of therectangular bar structure, 310, in FIG. 3, would short circuit the polesproducing the magnetic deflection field.

In principle, the vertical bars, 312, illustrated in FIG. 3, togetherwith their associated windings, 332 and 334, have been severed at thecentral symmetry-point of each of the bucking windings. Referring againto FIG. 14, the bucking windings associated with the cut-away upper barare labeled 1400, 1401. The windings that produce the focusing field arelabeled 1410. After severance it should be arranged that the samecurrent continues to pass through the resulting ‘half-windings’, 1400,1401, so that when a lens/corrector is used in lens mode each resultanthalf winding will produce half the ampere turns as the original windings332 and 334, illustrated in FIG. 3. Each element has three independentlywound excitation coils that, if necessary, can themselves be wound as acollection of independent coils, 330, such as those shown in FIG. 3, toallow the introduction of multipole correction fields. Just as in thestructure presented in FIG. 3 where the ampere-turns around the wholebar structure must integrate to zero, the symmetry of the independentelement array, 1430 and 1431, requires that along the length of eachelement the total magnetostatic potential must integrate to zero.

FIG. 14 illustrates the cross section of a quadrupole designed accordingto the above prescription. A ferromagnetic bar is located at the centerof each element. This bar need not have a cylindrical cross section, butthose skilled in the art will recognize that the cross-sectional areamust be adequate to avoid saturation. Three independent windingsections, 1400, 1401 and 1410, are wrapped around each bar. To allowmultipole generation and aberration correction the individual windingsections can themselves consist of a group of individually excited coilsas was illustrated in FIG. 3, item 330. Ferromagnetic extension tabs,420, introduced in the manner shown in FIG. 4, transfer themagnetostatic potential, generated along the length of the central steelbar, close to the boundary of the ribbon ion beam. The effect is tominimize the volume of magnetic field that must be produced and theneeded ampere turns. Also, to improve the spatial resolution of thelens/corrector fields at an ion beam boundary in the lens aperture.

Without reservations the bars and associated coil structures areenclosed within closed tubes, 1430, 1435, manufactured from a suitablenon-magnetic material having rectangular cross section. This enclosingtube structure permits the outside walls of the tube to be in vacuumwhile power leads to the coils and air or water cooling is readilyaccessible through the ends, 1460 and 1461.

A useful feature of the lens/corrector presented in FIG. 14 is while thetotal magnetostatic potential generated along each element mustintegrate to zero, it is not essential to pass equal currents throughthe windings within the elements 1430 and 1431. An unbalance in currentratio between the two elements changes the position of the neutral axisof the lens causing it to move in the Y-direction an introduce steeringof an ion beam about the X-axis.

Hydrogen Implanter

FIG. 15, a further embodiment of the present invention, shows theprinciples of a high current H⁺ implanter for implanting ions intolarge-diameter semiconductor wafers using the ion transport elementsdescribed earlier. A suitable ion source, 10, produces a ribbon array ofbeamlets, 12, with all beamlets having the same energy, between 10 keVand 100 keV. A multipole corrected diverging lens, 20, introducesdiverging angles into the array, 22, of beamlets to produce thenecessary ribbon width. A momentum-dispersing magnetic field, 30, withits B-field vector in the plane of the diverging beamlets andapproximately at right angles to the central beamlet of the array,deflects the ions at right angles to the plane of said ribbon beamallowing ions heavier than H⁺ to be collected into a cup, 40; thisarrangement eliminates deuterium and other molecular contributions. Asecond multipole-corrected lens, 50, collimates the array of thediverging beamlets and returns the beamlets to parallelism. A platensupports a wafer, 60, and uniformly scans it, across the beam. Thisnovel yet simple system employs no electromagnetic beam scanning. Theadvantages are short length, low cost, a simple optical path and smallfootprint.

FIG. 16 shows the manner in which multiple-use coils can be mountedalong a short section of one of the high permeability bars, 1617, toprovide the high magnitude ampere-turns that are needed for excitingsome deflection modes. It can be seen that continuous high-currentcapacity water-cooled coils, 1616, are wrapped as an under layerdirectly around a cylindrical magnetic core, 1617. Individuallyexcitable coils, 1618, as shown in FIG. 3 as items 330 and 340, alsosurround the high permeability steel bar, 1615, to provide focusing andaberration corrections. Individual steel tabs 420, transfer themagnetostatic potentials to the region near to the beam.

Any additional changes in the details, materials, and arrangement ofparts, herein described and illustrated, can be made by those skilled inthe art. Accordingly, it will be understood that the following claimsare not to be limited to the embodiment disclosed here, and can includepractices otherwise than those described, and are to be interpreted asbroadly as allowed under the law.

1. An apparatus for variable focusing of charged particles used forimplanting a plurality of charged particles into semiconductor materialsthereby modifying properties of said semiconductor materials, saidapparatus comprising: an upper magnetic core member; a lower magneticcore member spaced apart from said upper core member, said lower coremember being oriented with its axis substantially parallel to the axisof said upper core member; a plurality of focusing coil unitsdistributed along said upper core member and corresponding focusing coilunits distributed along said lower core member, each said focusing coilunit comprising a single continuous electrical circuit that surrounds anindividual core member; and a current controller configured to excitesaid plurality of focusing coil units distributed along said upper coremember and said corresponding focusing coil units distributed along saidlower core member such that the direction of the current in each of saidplurality of excited focusing coil unit distributed along said uppercore member is the opposite of the direction of the current in saidcorresponding focusing coil unit distributed along said lower coremember when viewed from one end of the upper and lower core members. 2.The apparatus of claim 1 wherein said plurality of focusing coil unitsare equally spaced.
 3. The apparatus of claim 1 wherein said pluralityof focusing coil units have an identical number of coil turns.
 4. Theapparatus of claim 1 wherein said current controller is configured forindependently changing the current passing through said focusing coilunits to produce a controllable magnetic field configuration betweensaid upper and lower magnetic core members.
 5. The apparatus of claim 1further comprising magnetic core extension units that are attached in alinear array to each of said upper and lower magnetic core members. 6.The apparatus of claim 1 further comprising two cross members, each incommunication with one end of each core member so as to form arectangular frame.
 7. The apparatus of claim 6 wherein said crossmembers comprise non-magnetic material.
 8. The apparatus of claim 6wherein said cross members comprise magnetic material.
 9. The apparatusof claim 8 further comprising at least one coil unit distributed alongeach of said cross members.
 10. The apparatus of claim 1 furthercomprising end coil units located on said upper core member between saidplurality of focusing coil units and each end of said upper core memberand end coil units located on said lower core member between saidplurality of corresponding focusing coil units and each end of saidlower core member.
 11. The apparatus of claim 10, wherein the current insaid end coil units located on said upper core member is in the oppositedirection from the current in said end coil units located on said lowercore member when viewed from one end of the upper and lower coremembers.
 12. The apparatus of claim 1, wherein the current in each ofsaid plurality of focusing coil units distributed along said upper coremember is in the same direction.
 13. An apparatus for variable focusingof charged particles used for implanting a plurality of chargedparticles into semiconductor materials thereby modifying properties ofsaid semiconductor materials, said apparatus comprising: an uppermagnetic core member; a lower magnetic core member spaced apart fromsaid upper core member, said lower core member being oriented with itsaxis substantially parallel to the axis of said upper core member; aplurality of focusing coil units distributed along said upper coremember and corresponding focusing coil units distributed along saidlower core member, each said focusing coil unit comprising a singlecontinuous electrical circuit that surrounds an individual core member;and a current controller configured to excite one of said focusing coilunits distributed along said upper core member and its correspondingfocusing coil unit distributed along said lower core member such thatthe direction of the current in said excited focusing coil unitdistributed along said upper core member is the opposite of thedirection of the current in said corresponding focusing coil unitdistributed along said lower core member when viewed from one end of theupper and lower core members.
 14. The apparatus of claim 13 wherein saidplurality of focusing coil units are equally spaced.
 15. The apparatusof claim 13 wherein said plurality of focusing coil units has anidentical number of coil turns.
 16. The apparatus of claim 13 whereinsaid current controller is configured for independently changing thecurrent passing through said focusing coil units to produce acontrollable magnetic field configuration between said upper and lowermagnetic core members.
 17. The apparatus of claim 13 further comprisingmagnetic core extension units that are attached in a linear array toeach of said upper and lower magnetic core members.
 18. The apparatus ofclaim 13 further comprising two cross members, each in communicationwith one end of each core member so as to form a rectangular frame. 19.The apparatus of claim 18 wherein said cross members comprisenon-magnetic material.